The miniature is 66 mm high and 54 mm across.
He was an effective naval captain for many years, even getting votes of thanks from both the House of Lords and, separately, the House of Commons.
After retiring from the Navy he was eventually appointed in 1789 to the highly rewarded post of Naval Commissioner of the Victualling Office. And in 1794 when George III made a royal visit to the ships at Spithead, he and his fellow Commissioner were both knighted (and three admirals were made baronets). He died in October of the following year.
For fuller information on his life, go to the Searchable Family Tree and look him up within Surnames.
The 2003 valuer of this miniature wrote: "Pair, probably husband and wife, she by Samuel Shelley, he possibly by Henry Edridge."